1. Field of the Invention
This invention relates to scanning electro-optical systems. More specifically, the present invention relates to methods and apparatus for extending the scan of an angularly scanning instrument.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof.
2. Description of the Related Art
A resonant oscillatory scan drive system is a simple yet effective scan drive system in that scan drive is effectuated by the pendulum like motion of a pivoted mass. This technique is often used in angularly scanning radiometers and electro-optical instruments because it requires the input of minimal additional energy (i.e. that sufficient to overcome inherent frictional forces) to maintain the oscillatory scanning motion at the resonant frequency.
As with other scanning techniques, the system scans across a scan line over an angular range of 56 degrees, for example, on either side of a zero scan angle (often referred to as the `nadir` position). Limiting the scanning motion at the end of scan positions may be accomplished by any means including the use of bumper stops or by designing the system so that the pivoted mass does not undergo excursions outside a designated range.
Whatever the technique employed for limiting the scan, a need has been recognized in the art for an angularly scanning system having a selectively extendible scanning range for secondary tasks. One common secondary task is that of calibration. Calibration typically requires the extension of the scanning range to bring a known target within the field of view of the system. Though generally desirable, this full-aperture end-to-end calibration feature has been difficult to implement in the several resonant oscillatory scan drive system approaches that have been investigated to date.
One approach, for example, involves the use of a constant peak amplitude oscillation that nominally substantially overscans the target area. This permits the radiometer instantaneous field of view (IFOV) to cover a calibration target at the end of scan position without blocking the image scene. Unfortunately, this approach forces a trade-off in radiometric resolution and efficiency. (Radiometric resolution may be measured in terms of signal-to-noise ratio (S/N) or the noise equivalent radiance (NEN) which is defined as the amount of radiation that must be received by the system detector assembly to produce an electrical signal equal to the system electrical noise.)
A second conventional approach for providing full-aperture end-to-end radiometric calibration of radiometer using a resonant oscillatory scan drive system involves the lowering of a calibration source into the scanned field-of-view at one end of the scan cycle, whenever calibration is required. This method may be cumbersome to implement in that the calibration target must be lowered and raised every time calibration is required. Also, the calibration target typically blocks part of the image scene so long as it is in place resulting in an undesirable momentary loss of data.
A third approach involves the momentary increase in the maximum scan angle of the system such that it overscans the target scene and looks at a calibration target when a calibration task is to be accomplished. This approach is undesirable for at least three reasons. First, it requires the addition of power to the scanning mechanism to increase the peak amplitude of the scan and again to reduce the peak amplitude to its original value. Secondly, the scan drive and affected subsystem must be designed to accommodate the additional stresses associated with the added power and the resulting increased oscillatory amplitude. Finally, the image scene data is adversely affected until the scan drive is returned to the normal scan amplitude operation.
Thus, there is a need in the art for an angular scan drive system having means for selectively extending the range thereof which does not compromise performance and which does not require a significant addition of power.